There is a growing demand in a cheap and reliable carrier of digital information for computers, video systems, multimedia, etc. This carrier should have a storage capacity in excess of 10.sup.10 bytes, fast access time, high transfer rate and long term stability.
Digital information is recorded at high density (about 10.sup.8 bytes per square inch) using optical and magnetic methods. Optical and magneto-optical disks, magnetic disks and magnetic tapes are most popular carriers of digital information. Optical methods have advantages over magnetic ones because of less restricted requirements to the components and environment. Parallel writing of information at mass production is another advantage of the optical memory. The information is usually written as local variations of thickness, refractive index or absorption coefficient of the media.
The optical disk capacity is diffraction limited by a value of (disk area .lambda..sup.-2) bits because only one binary value is stored in a diffraction limited pixel. Quadrupled capacity can be gained using "super resolution" at fractions of wavelengths. High density of information is received when 3-5 bits are stored in a single pit as a small variation of the pit length around diffraction limit. Precision optical, mechanical and electronic components as well as high quality media are required to realize this method.
The stacks of two or more disks is a way to increase the capacity of digital data carrier. Some problems of disks stack methods are listed below. They are the multiple reflections between the reflective surfaces, power losses at each information plane during the propagation of reading and reflected beams to and from the internal layers, interference of the light reflected from different layers, and beam distortions due to the optical aberrations. The aberrations appear when the optical path within the storage media is changed to read different information planes. High quality optical adhesives are required to assemble stack of disks, having on aberrations, bubbles, separations, inclusions as well as no mechanical, thermal and chemical impact on the disks. The information capacity of multi stack disks is limited in practice around 10.sup.10 bytes. These discs are composed of 2 structures of 2-3 information layers, the structures are attached together by back sides to double capacity of the disk to the value around 2.multidot.10.sup.10 bytes.
Three dimensional (3-D) recording can dramatically increase the capacity of storage device. An optical memory enables to store the binary-stated information into a storage medium at very high density, each binary bit occupying a space only about wavelength in diameter, .lambda..sup.-3.about. 10.sup.12 bits/cm.sup.3. When practical limitations are taken into account it reduced to 10.sup.10 -10.sup.11 bits/cm.sup.3.
The 3-D writing and reading was reported (J. H. Stricler, W. W. Webb, Optics Lett., 16, 1970, (1991); H. Ueki, Y. Kawata, S. Kawata, Applied Optics, 35, 2457, (1996); Y. KAWATA, R. Yuskaitis, T. Tanaka, T. Wilson, S. KAWATA, Applied Optics, 35, 2466, (1996);), using local changes of refractive index of the optical media. These local variations of refractive index result in the birefringence and variations of polarization of the reading beam transmitted thought the media. The variations were measured and can be interpreted as a binary code. The signal is very weak requiring high power laser and highly sensitive detectors.
The 3-D regular structure of the information carrier acts as a material at a macro scale introducing the non informative depolarization and defocusing of the transmitted beam.
The variations of the refractive index of the media introduce the phase modulation located in the adjacent layers, diffraction and power losses. The measurement of the transmitted beam is conducted requiring two optical head (transmitting and receiving) from both sides of the carrier.
This optical solution is very complicated and expensive requiring simultaneous alignment of the heads to a diffraction limited spot, especially taking into account the variation of the required optical path, medium inhomogeniouty, and carrier/heads movement perturbations. The data recording is possible only by the laser utilization--thus it does not allow to implement cheap replication method, like mask lithography.
Solidification/polymerization process results in non-controllable material deformation during the recording procedure. Afterwards, the volume induced strengthens may lead to the recorded site movements and information distortion. Thus, all mentioned above drawbacks will put obstacles in the way of converting this approach to be realized into the practical 3D-memory device.
First device for reading the information from the multilayer fluorescent medium was suggested by Russell, U.S. Pat. Nos. 4,090,031 and 5,278,816, and was based on utilization of optical data layers with different color dyes or different fluorescent materials and selectively positioning of corresponding color filters in front of the light detector.
Further, the fluorescent 3-D memory and apparatus for retrieving the information was suggested by D. A. Pathenopoulos and R. M. Rentzepis, Science, 245,843, (1989).[5]., R. M. Rentzepis, U.S. Pat. No. 5,268,862. The active material contains the photo chromic molecules having two isomeric forms. First isomeric form "A" is not fluorescent, it has absorption bands for UV radiation, and it is transferred to the second form "B" under the two visible photon absorption. The form B absorbs the two photon of reading radiation and fluoresces in the infrared range.
The information is retrieved from the medium by a two-photon absorption process, and corresponding fluorescence at the point where two focused beams are crossed at the region having dimensions of .lambda..sup.-3. Each beam should be formed by a picosecond or femtosecond pulse of light to provide the intensity required for both writing and reading processes. This also means that two pulses should overlap in a time domain. The Rentzepis patent [6], was the first implementing the photo chrome materials for 3-D optical memory, and describing the principles and methods of two-photon rewritable 3-D apparatus.
Accordingly, this approach has also a series of drawbacks, which will hardly permit it to be practically realized. First, the two-photon approach requires extremely high intensity laser pulses, I.sup..about. 10.sup.12 -10.sup.13 W/cm.sup.2, which in turn requires the femtosecond pulse width Ti:Saphhire lasers. Second, the .mu.m-sized intersection of two focused laser beam required for 3-D reading would be very difficult or even impossible for practical realization. Third, the reliable, stable photochromic material which may withstand multiple writing/erasing/reading cycles at a room temperature and posses the optical properties compatible with the existed miniature (diode) laser sources does not yet exist. Another problem is a long time required for writing of the information into the disk. The required time is about 10.sup.5 sec, if optimistic information writing rate is 10.sup.6 bits/sec. This makes the described solution to be very expensive even for the mass production. Thus, all mentioned problems make the described method impractical for optical memory device.
There are many different physical principles based on transmission, holography, polarization e.t.c. for recording and reading the optical information, but only two of them based on the reflectivity and fluorescence provide simple, convenient and reliable optical pick-up, where the same objective lens is used for both focusing the incoming light to the layer and collecting the out coming light for detection.
The 3-D optical data reading by one focused laser beam is inevitably followed by fluorescence of a large number of fluorescent cells from non addressed layers confined within the conical surface of the focused laser beam. There are several main drawbacks typical for all existed or described in a number of patents optical apparatus to readout from 3-D or multilayer disk structure:
1. Different intensity of reading laser at different information layers. PA1 2. Different detected signal value coming from different information layers. PA1 3. Strong noise from non addressed layers due to scattering and/or multiple reflection. PA1 4. Dramatic reduction of Signal-to-Noise ratio. PA1 5. Reading beam quality distortion associated with non uniform, refractive index distribution in a thick medium. PA1 6. Spherical aberrations associated with thick medium and variable focus depth requirement. PA1 (1) only one focused to the diffraction-limited spot size reading laser beam, PA1 (2) optical as well as electronic for selective detection of single bit information from the addressed layer and separation from the noise coming from all non addressed layers, which includes PA1 (3) optical means to provide the auto tracking and auto focusing error correction system which is suitable for non reflective, transparent fluorescent media, which based on fluorescence signal from continues tracks or fluorescent cells and photo diode array (4-section photo diode).
The obvious disadvantage of the fluorescent principle is low coupling efficiency: while reflected signal value can easily reach 50-80% of incident light, the fluorescence light which is collected by the lens with NA=0.6 can not overcome 10% of incident light. Usually it is much less taking into account the pit absorption rate and fluorescence quantum efficiency of material. However this situation is dramatically changed when the multiple optical data layer system is considered. It would be demonstrated later that even for three data layer disc the detected signal value from reflective and fluorescent media becomes the same .sup..about. 1% of incident light power. While the reflectivity signal will be completely lost due to the multiple reflection, and unavoidable attenuation during it's propagation through the reflective layers, the fluorescence output can be made completely free of noise.